The synaptic organization of the dorsal raphe nucleus of the rat will be studied using intracellular recording techniques, intracellular staining of recorded neurons with horseradish peroxidase (HRP) and labeling of the autoradiographic serotonergic neurons. The physiological studies entail the recording of postsynaptic potentials resulting from antidromic stimulation and the stimulation of afferents to the dorsal raphe. Experiments are proposed to separately activate the various synaptic inputs and to test if they involve mono- or polysynaptic pathways in order to determine how the neurons of the dorsal raphe nucleus and their inputs are synaptically interconnected. Injection of HRP will permit light microscopic description of the morphology of dorsal raphe neurons. When correlated with the physiological responses and autoradiographic labeling as to neurotransmitter, this analysis will produce a description of the number and character of cell types found in the dorsal raphe.